The itinerary has changed, and the visa is not valid enough. Can it be extended?

Yes, but under special circumstances. With the approval of the entry visa officer, the validity of the visa can be extended for up to 3 months if the same purpose of the visit can be provided. Otherwise, you should apply for another visa.

Visa refers to the procedure of endorsement and seal of passports or other travel documents held by domestic or foreign citizens by the sovereign organs of a country to show that they are allowed to enter, exit or transit. It can also be said that it is an endorsement certificate issued to them. Generally speaking, a visa is a document issued by a country's entry-exit administration (such as immigration bureau or its embassy or consulate abroad) to approve the entry of foreign citizens.